A child looks on as migrants wait in line inside a registration camp in Presevo, Serbia, January 20, 2016. REUTERS/Marko Djurica

Governments and aid agencies along the route have laid on heated tents and mobilized trains and buses to support the flow of migrants, most of them refugees from the war in Syria winding across the Balkan peninsula.

But the sheer numbers – though down from a summer peak of some 10,000 to just under 2,000 per day – mean many spend nights sleeping on tent floors.

A Reuters photographer saw children crying from the cold as they walked or were carried several kilometers across the Macedonian-Serbian border to waiting buses.

The United Nations and aid agencies warned on Tuesday that children were particularly at risk given their lack of adequate clothing or access to sufficient nutrition.

A spokesman for the U.N. children’s agency UNICEF said the risk of children freezing to death was “clearly very, very high.”

Most migrants wore jackets and sneakers; some had hats and gloves, and many were wrapped in gray blankets handed out by the United Nations refugee agency (UNHCR).

“It’s too cold, but what can I do? I’m wearing everything I have but it’s still too cold,” said a 22-year-old man at the town of Sid on the Serbian-Croatian border. He gave his name as Amr and said he was from the Iraqi town of Fallujah, where Islamic State militants hold sway.

On the highway in Serbia, hundreds of migrants received hot soup, tea and gloves from aid groups at a disused motel that has been turned into a refugee camp.

More than a million people fleeing war, poverty and repression in the Middle East and Africa reached Europe’s shores last year, most heading for Germany.

Aid agencies expect a similar number this year, testing the willingness of a divided Europe to take them in and putting unprecedented strain on the continent’s commitment to a Schengen zone of open borders.